Colloquial term describing the principle that already vested rights usually cannot be taken away by changes in law or regulation (and sometimes contracts). Thus a classic example of a grandfather clause arose in the United States when states raised their legal age for the purchase and consumption of alcohol from 18 years to 21; the new laws generally provided that those who were over the age of 18 as of the date that the new drinking age came into force, could continue to buy and consume alcohol.
While grandfathering usually applies to reduction in or removal of vested rights, it does not normally apply to the expansion or extension of rights. Thus copyright term extensions were often granted under the TRIPS, even to copyright authors whose rights were already lapsed when the TRIPS extended the copyright term in a relevant jurisdiction – and typically, the extended rights accrued to the original authors, composers, artists or their estates and not any existing assignees or licensees of the original term(s.)